Current events underscore the need for an improved and inexpensive analytical device capable of rapidly, reliably, and accurately detecting explosives, toxic chemicals and biologics, chemical warfare agents, and other harmful materials. Spectrometers based on ion mobility have been previously developed to serve this purpose, but technological improvements are still needed to reduce detection time, increase sensitivity, enable environment adaptability, reduce noise interference, improve prediction accuracy, and reduce power consumption.
Conventional spectrometers typically employ either ion-mobility spectrometry (IMS) or differential ion mobility spectrometry (DMS) as the broad method by which they identify compounds in a sample gas taken from an ambient environment. Conventional IMS devices, which are well known in the art, are based on time-of-flight (TOF-IMS) analysis. TOF-IMS identifies compounds by measuring the time it takes ions to travel through a drift tube, usually on the order of milliseconds, from a shutter-gate to a detector electrode. The drift time is dependent on the mobility of ions in a linear, low electric field, which accelerates the ions in the drift tube. The measured drift time is characteristic of the ion species present in the sample. In IMS systems, an ion's mobility coefficient is independent of the electric field strength but its velocity is proportional to the electric field strength.
Though typical IMS and DMS based devices share many of the same system components (inlet system, ionization source, readout electronics) DMS devices operate very differently. DMS devices characterize chemical substances using differences in the gas phase mobilities of ions in alternating, high-frequency, asymmetric electric fields. Ions are separated as they are carried by drift gas between two-parallel plates or filter electrodes. At higher electric field strengths there is a nonlinear dependence of ion mobility. A high-frequency asymmetric electric field is produced by applying a high-frequency asymmetric differential potential between the plates. An equivalent field could be produced by applying a differential potential to both plates relative to ground, or to one of the plates with the other grounded. This applied field, referred to as the separation or dispersion voltage, causes ions to oscillate perpendicular to the gas flow. Some ions traverse the filter electrodes, while others gradually move towards one of the electrodes and eventually collide with an electrode, which neutralize the electric charge in such ions. Only ions with a net velocity or differential mobility of zero transverse to the applied electric field will pass through the electrodes.
The net migration of the ions can be corrected with a compensation voltage (Cv), which is a weak dc voltage superimposed on the high-frequency, asymmetric electric field, to correct the path ions travel so they do not move towards an electrode.
Various techniques of DMS have been developed, including field ion spectrometry (FIS), transverse field compensation ion mobility spectrometry, ion non-linearity drift spectrometer, field asymmetric ion mobility spectrometry (FAME), and radio frequency ion mobility spectrometry (RFIMS), among others.
However, current devices that use DMS to identify compounds, although improved over conventional TOF-IMS systems, still have deficiencies that must be improved, like reductions in detection time, noise, and power consumption; increases in sensitivity; and improvements in environment adaptability, and prediction accuracy.